1. Field of the Invention
The present invention relates to a calibration apparatus and a method of calibrating a radiation sensor in a lithographic apparatus, and is concerned more particularly, although not exclusively, with calibration of a lithographic apparatus designed to be used with radiation having a wavelength in the Extreme Ultra-Violet (EUV) range and wherein the lithographic apparatus comprises a sensor for measuring the radiation dose falling on a substrate.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that circumstance, a lithographic patterning device, which is alternatively referred to as a “mask” or “reticle,” may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (i.e., resist).
In general, a single substrate will contain a network of adjacent target portions that are successively exposed. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion in one go, while in so-called scanners, each target portion is irradiated by scanning the pattern through the projection beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
The term “patterning device” used herein should be broadly interpreted as referring to a device that can be used to impart a projection beam with a pattern in its cross-section such as to create a pattern in a target portion of the substrate. It should be noted that the pattern imparted to the projection beam may not exactly correspond to the desired pattern in the target portion of the substrate. Generally, the pattern imparted to the projection beam will correspond to a particular functional layer in a device being created in the target portion, such as an integrated circuit (IC).
The patterning device may be transmissive or reflective. Examples of patterning means include masks, programmable mirror arrays, and programmable LCD panels. Masks are well known in lithography, and include mask types such as binary, alternating phase-shift, and attenuated phase-shift, as well as various hybrid mask types. An example of a programmable mirror array employs a matrix arrangement of small mirrors, each of which can be individually tilted so as to reflect an incoming radiation beam in different directions; in this manner, the reflected beam is patterned.
As noted above, during the manufacturing process employing a lithographic apparatus, the pattern of the patterning device (e.g. mask or reticle) is imaged or exposed by having radiation impinge onto the patterning device and eventually to the substrate that is at least partially covered by a layer of radiation-sensitive material (e.g. resist). The effect on the resist of radiation falling thereon is highly sensitive to the dose of radiation. It is, therefore, necessary to measure the radiation falling on the substrate target portion. This is achieved by the provision of a sensor (e.g. a photodiode) on the stage onto which the substrate is held.
An important parameter in lithography is the size of features of the pattern applied to the substrate. It is desirable to produce apparatus capable of resolving features as small and close together as possible. A number of parameters affect the available resolution of features, and one of the most important of these is the wavelength of the radiation used to expose the pattern.
It is anticipated that the use of EUV lithography will enable the manufacture of feature sizes below 32 nm using radiation with an EUV (Extreme Ultra Violet) wavelength between 5 and 20 nm, and typically 13.5 nm. Radiation at this wavelength is absorbed in most materials, and the substrate is enclosed within a vacuum chamber to prevent attenuation of the radiation beam. The sensor referred to above is also enclosed within the vacuum chamber.
A serious problem with current EUV lithography systems is contamination inside the vacuum chamber. All surfaces inside the vacuum chamber include molecules which are only weakly bonded, and contaminants are emitted from these surfaces when the chamber is evacuated. In particular, when the layer of resist provided on the substrate is illuminated by radiation, particularly large levels of contaminants are released. This occurs because of the energy of EUV photons, which is higher than chemical bond strengths.
These contaminants typically include hydrocarbons, water and/or sulphur, although it will be appreciated that other materials may also be released. Because the contaminant molecules are released into a vacuum they rapidly disperse throughout the vacuum chamber and build up over time on all surfaces within the vacuum chamber, including the active surface of the radiation sensor. When a surface with contaminant build-up is exposed to EUV radiation (e.g. the active surface of the radiation sensor) the contaminants are “baked on” by the EUV exposure. Further contamination occurs by oxidation of illuminated surfaces within the vacuum chamber.
The contamination of the radiation sensor surface leads to a degradation in the performance of the sensor. The sensitivity of the sensor thus decreases over time, and it becomes unreliable for measuring the absolute radiation dose received at the substrate.
In order to overcome this problem, radiation sensors are normally calibrated at regular time intervals using external reference sensors. However, in the case of EUV systems, these external reference sensors are also subject to contamination. Even if they are contained within a separate vacuum chamber contaminants will be baked on each time the reference sensors are used to measure radiation. Thus over many calibrations the contamination even on the reference sensor will build up. Because the amount of contamination on the reference sensor is then unknown, an absolute calibration of the radiation sensor becomes impossible.